The present invention is generally directed to solid state image sensors and, in particular, to a solid state image sensor having a plurality of sensing cells on an insulating substrate with the individual cells consisting of a photo-sensitive sensor and a high speed non-photo sensitive thin film transistor.
Heretofore, cathode ray tubes have been used to convert optical images to electrical signals. The cathode ray tubes have the twin disadvantages of being short lived an large. Recently, however MOS type CCD type image sensors have been developed.
Reference is made to FIG. 1 wherein a monochrome picture MOS image sensor in accordance with the prior art is depicted. The prior art image sensor is composed of an N.times.M matrix of sensing cells 3. Each sensing cell 3 consists of photo-diode 2 and a MOS transistor 1 having a gate electrode 15, a drain 13 and a source 12 to read out the charge accumulated on photo-diode 2. Gate electrode 15 of MOS transistor 1 is connected to one of a plurality of horizontal scanning lines H.sub.1 to H.sub.M connected to the output of a shift register 7 which selects one of horizontal scanning lines H.sub.1 to H.sub.M. Source 12 of MOS transistor 1 is connected to one of the plurality of vertical video signal lines V.sub.1 to V.sub.N. Photo-diode 2 is connected to drain electrode 13. Each vertical video signal line V.sub.1 to V.sub.N is connected to a video signal line V.sub.s by a corresponding switching transistors S.sub.1 to S.sub.N. Shift register 6 selects which video signal line V.sub.1 to V.sub. N to energize and switches on the appropriate switching transistor 4 which places the electric signal on the chosen vertical video signal line V.sub.1 to V.sub.N on to video signal line V.sub.s. Shift register 6 energizes all of the switching transistors
S.sub.1 to S.sub.N in order during one horizontal scan. When a horizontal scanning line H.sub.1 to H.sub.M is selected by shift register 7 it has the effect of placing the photo-excited electric charges of photo diodes 2 onto vertical video signal lines V.sub.1 to V.sub.N. The effect of this system is to place on video signal line V.sub.s in serial form the photo-induced electric charges of an entire line of sensing cells 3.
Reference is next made to FIG. 2 wherein a sensing cell formed from a monolithic semiconductor substrate in accordance with the prior art is depicted. A P-type well is formed in an N-type monocrystalline silicon substrate 10 with a field oxide layer 16 formed thereon. Diffused regions 12 and 13 which serve as a source and drain for the transistor are formed. An aluminum connector 14 and a gate electrode 15 to receive a horizontal scan signal is included. An N-type drain region 13 and P-type well 11 constitute photo-diode 2. Photo diode 2 is usually biased in the reverse direction and the charge to be stored is always kept constant in the depletion layer of the PN junction 11-13. When transistor 1 is turned on through gate electrode 15 the photo induced charge flows out onto the vertical video signal line V.sub.1 to V.sub.N corresponding to sensing cell 3. The amount of charge stored in photo- diode 2 varies in proportion to the intensity of light incident thereon. Therefore, serially reading out the amount of charge in each cell on a horizontal line of sensing cells 3 allows the conversion of a line of an image to an electrical signal.
However, the devices of the prior art method have several major problems. In addition to photo-diode 2 being sensitive to light, the remaining semiconductor portions of sensing cell 3, specifically transistor 1 are photo-sensitive. This causes blooming and smear. When the incident light is strong, too much charge to be stored on photo diode 2 is present and the excessive charge is dissipated into adjacent sensing cell 3 and adjacent vertical video signal lines. This causes the production of abnormal vertical lines on the display. This phenomenon is called blooming. In addition, when the incident light strikes the lower portion of sensing cell 3 other than photo-diode 2 this invites the generation of the carrier in the depth of sensing cell 3. As a result, the display appears smeared with white. This phenomenon is called a smear. It is possible to reduce blooming and smear with improved cell construction including stricter patterning of the sensing cells or by complicating the peripheral portion of the external sense-amplifier. However, with the prior art constructions it is not possible to completely eliminate blooming and smear completely. This results in deteriorated power of the solid state image sensor and increased overall costs for manufacture.
Recently, there has been a demand for a facsimile machine or intelligent duplicator based on a line sensor at a low price and with good image processing. At present, facsimile machines can be bought for less than $1,000. Facsimile machines are composed of a read out portion, a printing portion and a communication system. The development of thermal print heads and the improvement of large scale integration techniques have lowered the manufacturing costs associated with the printing and communications components. However, the complicated optical systems required for the read out portion and the manufacturing of the sensor itself have remained high. This has kept the overall costs of manufacturing a facsimile quite high. It is therefore desirable to develop a high powered read out portion at low cost. A sophisticated powerful machine with intelligent functions such as a facsimile machine, a copy machine and a printer can be sold inexpensively if the read out portion can be manufactured at low cost.
One attempt to lower the manufacturing cost of the read out portion was a contact type sensor. The contact type sensor has recently been under consideration. However, these sensors have poor reliability, insufficient characteristics and high manufacturing costs which rule out mass production because of complication of the external treatment.
In conventional image sensors with single crystal substrates the signal to noise ratio is very poor. In addition the flaws in crystallization cause poor uniformity of response thereby making a clear gray scale display difficult to achieve.
On the other hand, solid state area image sensors of the MOS type and CCD type utilizing monocrystalline substrates have entered the market.
Although solid state image sensors of the type noted above have been used to sense images, it is noted that there is a need for a solid state image sensor which improves the signal to noise ratio and uniformity of the sensing cells so as to eliminate blooming and smear completely. Accordingly, a solid state image sensor which can eliminate blooming and smear completely is desired.